Issue 50

A.G. Lekatou et alii, Frattura ed Integrità Strutturale, 50 (2019) 423-437; DOI: 10.3221/IGF-ESIS.50.36 426 connected to the Gill AC potensiostat/galvanostat by ACM Instruments. The corrosion current densities were determined by Tafel extrapolation, by conforming to several criteria analytically documented in previous efforts [36,37]. Reverse (or cyclic) polarization at a scan rate of 10 mV/min was carried out to evaluate the susceptibility of the rebars to localized corrosion. This technique is based on the concept of the hysteresis loop formed by the current density difference between the forward and reverse scans at the same potential. A negative hysteresis loop (i.e. a loop formed by anodic reverse scans corresponding to higher current densities than the current densities of the forward scans) signifies occurrence of localized corrosion [37]. Two types of electrolyte were used: a) The first electrolyte contained (1.8-1.8x/100) g of agglomerates of Ca(OH) 2 per l of an AR mimicking solution (pH=3.1). FA was added in the solution in amounts varying from x=0 to 25 wt.% of {Ca(OH) 2 +FA}. The pH of the electrolyte ranged from 4.5 (0% FA) to 6.9 (25% FA), postulated to represent concrete pores having become unsaturated due to acid attack. Considering that a pH acidity of 5.5-6.0 may be the limit of tolerance of high quality concrete in contact with various acids [19], this type of electrolyte can simulate the case of corroded cover concrete that exposed the reinforcement to direct AR attack possibly through cracks or pores. (316L bars were also tested in this electrolyte, for comparison reasons.) b) The second electrolyte consisted of an acid rain mimicking solution of pH=3.1 containing 1.8 g/l of a mixture of Ca(OH) 2 powder and FA, the latter in amounts varying from 0 to 25 wt.% of the dry mixture. The initial pH of the electrolyte was 11.7-11.8 irrespectively of the FA content. Considering that water saturated with Ca(OH) 2 (1.8g/l of H 2 O) having a pH of ~12.6, simulates a concrete pore solution [38], a slightly acidified (due to AR) solution of Ca(OH) 2 (mixed with varying amounts of FA) is postulated to represent new concrete having been subjected to an AR attack that is expected to have a mild effect on the steel reinforcement. In the case of the alkaline electrolyte, only anodic polarization was carried out. In the case of the acidic electrolyte, cathodic polarization preceded anodic polarization, in order to attain even more aggressive conditions due to reduction reactions of surface oxides. Here it is noted that the interaction of the above electrolytes with the steel cannot simulate the kinetics of AR attack to rein- forced concrete. Nevertheless, the results presented in the following sections intend to clarify whether the effect of FA on the rebar corrosion has a consistent trend regardless of the severity of attack and also to provide an understanding of this trend. The acid rain mimicking solution had the following composition in g/l of H 2 O: H 2 SO 4 : 0.032, HNO 3 : 0.015, (NH 4 ) 2 SO 4 : 0.046, Na 2 SO 4 : 0.032, NaNO 3 : 0.021 and NaCl: 0.084 [39]. The microstructure of the polarized 304L specimens (cross-sectioned at ribs by a diamond saw and polished by standard metallographic procedure) was inspected by Scanning Electron Microscopy (SEM)/Energy Dispersion X-ray spectroscopy (EDX) at the JeoL JSM 6510 LV system equipped with an Oxford Instruments X- Act EDX analyzer. Salt spraying and tensile property evaluation Concrete cubes reinforced with 304L rebars underwent salt spray testing for 4 m in a Vötch chamber (5 wt.% NaCl, 35 o C). The stainless steel/concrete/salt fog junction was coated by epoxy glue. Uniaxial tensile testing of the rebars was performed before and after 2 m and 4 m of salt spraying in a Galdabini 100 kN Universal Testing Machine (ASTM E8/E8M-09, ambient temperature, cross-head speed: 1.30 mm/min). The modulus of elasticity of the embedded, before and after salt spraying, was determined by ultrasound testing (Pulse- echo technique). In the pulse-echo mode, the sound is reflected back to the device; hence, the transducer performs both the sending and the receiving of the pulsed waves. This technique determines the elastic modulus through measurements of the velocity of propagation of the longitudinal ultrasonic waves and the velocity of propagation of the shear ultrasonic waves in the specimen. These measurements are based on the principle that ultrasonic waves are stress waves propagating in materials when introducing small elastic deformations (in the order of 10 -7 mm) of the material. Hence, the dynamic moduli of a material can be determined by using the wave propagation equations according to the linear elasticity theory. These equations are analytically described in [40]. The experimental setup for the velocity measurements included piezo- electric transducers (x-cut: longitudinal wave generation and y-cut: shear wave generation). The ultrasonic acquisition and analysis were performed by the UTWin software. Honey was used as a couplant between the transducer and the specimen. The ultrasonic frequencies of the longitudinal and shear waves were 10 MHz. R ESULTS AND DISCUSSION Cyclic polarization in the low pH electrolyte he cyclic polarization behavior of 304L rebars during immersion in an electrolyte containing Ca(OH)2, acid rain simulating solution and fly ash at different contents at mildly to slightly acidic pHs (4.5, 5.4, 6.1, 6.5, 6.9 at 0, 10, 15, 20, 25 wt.% FA, respectively) and alkaline pHs (11.8, 11.7, 11.7, 11.8, 11.7 at 0, 10, 15, 20, 25 wt.% FA, respectively) is shown in Figs.1(a,b), respectively. T